Stepped acoustic impedance-matching transformer for very narrow channel acoustic traveling wave lens waveguide

ABSTRACT

An acoustic impedance-matching transformer includes a tapered acoustic energy transmission (metal) block, having an acoustic impedance less than that of a piezo-electric transducer in a metric direction toward that of the acoustic propagation medium of the waveguide, and by an amount that allows tailoring of the relative spatial dimensions of mating surfaces of the transducer and the block, so as to provide efficient broadband coupling of acoustic energy. The surface area of the end face of the block engaging the transducer is a corresponding fraction of the area of the transducer. The tapered block focuses acoustic energy toward the input aperture of the waveguide channel. The taper is determined in accordance with difference between the acoustic impedance of the aluminum block and that of a second acoustic wave propagation element, such as a quarter-wave section of plexiglass, interposed between the waveguide channel and the tapered block.

FIELD OF THE INVENTION

[0001] The present invention relates in general to laser beam scanningsystems, and is particularly directed to a acoustic impedancetransformer, which is interposed between a relatively large sized,electrically driven piezoelectric transducer and an acousto-opticmedium. The acoustic impedance transformer is configured to effectivelymatch the impedance of the acoustic transducer with that of the acousticwave propagation medium of the acousto-optic waveguide.

BACKGROUND OF THE INVENTION

[0002]FIG. 1 diagrammatically illustrates the configuration of a guidedacoustic travelling wave lens device—one that employs a relativelynarrowly dimensioned traveling wave channel—as comprising a laser 10,the optical beam output 11 of which is focussed by a cylindrical lensarrangement 12 and deflected by a mirror 13 onto an acousto-optic beamdeflector 14, to which an RF input signal is applied. Theacousto-optically modulated beam is then reimaged by a furtherspherical—cylindrical lens arrangement 15 onto a traveling lens cell 16,than contains a traveling wave lens transport medium 17 and a travelingwave lens launching transducer 18. The scanned beam is then imaged ontoan image collection medium, such as a photographic film 19.

[0003] In a number of applications, the acousto-optic waveguide may beconfigured as a reduced height, guided acoustic travelling wave lens(ATWL) waveguide device, such as that diagrammatically illustrated at 30in FIG. 2. In this type of acoustic wave guide architecture, a first end32 of the waveguide has an acoustic wave input aperture 34 (to which anacoustic wave-launching piezo-electric transducer is coupled), at aninput end of a relatively narrow (fluid-containing) channel 36, having across-section of width w and height h, where w>>h.

[0004] For a non-limiting illustration of examples of documentationdescribing such guided acoustic traveling wave lens devices, attentionmay be directed to an article entitled: “Optical Beam Deflection UsingAcoustic-Traveling-Wave Technology,” by R. H. Johnson et al, presentedat the SPIE Symposium On Optical, Electro-Optical, Laser andPhotographic Technology, August 1976, FIG. 6 of which corresponds toFIG. 1, above, an article entitled: “Guided acoustic traveling wave lensfor high-speed optical scanners,” by S. K. Yao et al, Applied Optics,Vol. 18, pp 446-453,February 1979, and the U.S. Pat. No. 3,676,592 toFoster.

[0005] In a reduced height guided wave device, because the acoustictransmission properties of the acoustic propagation medium (fluid)within the waveguide channel 36 are considerably different from those ofthe transducer being used to launch the acoustic wave into thewaveguide, there is a substantial acoustic impedance mismatch betweenthe transducer and the waveguide. Indeed, the acoustic impedance of thewaveguide may be on the order to twenty or more times that of thetransducer.

[0006] In such a circumstance, in order to provide significant energycoupling from the transducer to the waveguide's acoustic propagationmedium (e.g. water), the transducer must be allowed to resonate to verylarge internal power. This causes two problems. First, the acoustictransducer is prone to failure, as the result of the very substantialacoustic stresses required. Second, the bandwidth is limited.

SUMMARY OF THE INVENTION

[0007] In accordance with the present invention, this electrical andacoustic impedance mismatch problem is successfully obviated by means ofan acoustic impedance-matching transformer that is inserted between theacousto-optic medium and a piezo-electric transducer. The acousticimpedance-matching transformer is configured to effectively match theacoustic impedance of the acoustic transducer with that of theacousto-optic medium.

[0008] For this purpose, the acoustic impedance transformer isstructured as a combination of steps and tapers, in the form of acascaded series of acoustic propagation elements of successivelydecreasing acoustic impedance. At each interface between adjacentelements, the abutting surfaces or the elements are physicallyconfigured to provide an effectively acoustic impedance match betweenthe elements, and thereby an efficient coupling of the acoustic energyfrom an element of relatively higher acoustic impedance material to anelement of relatively lower acoustic impedance material. Over the lengthof the transformer, this sequential ‘stepping’ of the acoustic impedanceand configurations of the abutting surfaces of successively adjacentelements operates to effectively match the acoustic impedance of thepiezo-electric transducer to that of the liquid acoustic traveling wavelens.

[0009] The first acoustic wave propagation element of the transformercomprises a relatively dense, acoustic energy transmission material,such as a metal (e.g., aluminum) block. This first element has anacoustic impedance that differs (or is stepped down) from that of thepiezoelectric transducer in a metric direction toward the relativelysmall acoustic impedance of the acoustic propagation medium (water)within the ATWL waveguide. In order to compensate for this difference inthe acoustic impedances of the materials of the transducer and thealuminum block, the dimensions of the two materials in the direction ofthe step must be smaller than the acoustic wavelength in the respectivematerial. To this end, the spatial dimensions of the surface of theblock are larger than those of the abutting of the wave-launchingsurface of the transducer, and by an amount proportional to the inverseratio of their respective acoustic impedances, whereby the resultingacoustic-coupling interface provides broadband efficient coupling ofacoustic energy therebetween.

[0010] Although such dimensioning of the engaging faces of thepiezo-electric transducer and the aluminum block ensures efficientcoupling of acoustic energy from the transducer into the aluminum block,the cross-section of the aluminum block at its acoustic energy receivingface adjoining the piezo-electric transducer is still larger than thatof the acoustic wave coupling aperture of the waveguide channel. Also,the acoustic impedance of the aluminum block is considerably larger thanthat of the waveguide channel.

[0011] These differences are compensated by tapering the aluminum blocktoward the aperture of the waveguide channel, so as to focus theacoustic energy to a high acoustic energy density value at a reducedcross-section end face adjacent to the acoustic input aperture to thewaveguide, and by providing a second transformer element between thereduced cross-section end face of the tapered aluminum block and theacoustic input aperture to the waveguide. The extent to which the firsttransformer block is tapered is determined in accordance with differencebetween the acoustic impedance of the aluminum block and that of thesecond acoustic wave propagation element, such as a section ofplexiglass or the like, so as to provide a compensating abutting surfacearea mismatch therebetween, as in the case of the piezo-electrictransducer and the input end of the tapered aluminum block.

[0012] The plexiglass section has a cross-section corresponding to thatof the acoustic input aperture of the waveguide. Since plexiglass has anacoustic impedance that is approximately twice that of water, it isconfigured as a quarter-wave plate section, so as to provide an acousticimpedance match between its interface with the reduced cross-section endof the tapered aluminum block and the waveguide.

[0013] The combined effect of the increased acoustic power density atthe exit end of the aluminum block, the increased cross-sectional areaof the plexiglass quarter-wave plate, and the ratios among the acousticimpedance parameters of the mutually adjoining aluminum block,quarter-wave plate and waveguide channel thereby provides an efficient,broadband coupling into the waveguide channel of acoustic energyoriginally launched from the piezoelectric transducer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 diagrammatically illustrates the configuration of a guidedacoustic travelling wave lens device;

[0015]FIG. 2 diagrammatically illustrates a conventional acoustictraveling wave lens waveguide of a fixed narrow rectangular channelcross-section of fixed width and height;

[0016]FIG. 3 diagrammatically illustrates an acoustic impedance-matchingtransformer in accordance with the present invention; and

[0017]FIG. 4 diagrammatically illustrates a mounting arrangement for theacoustic impedance-matching transformer of FIG. 3.

DETAILED DESCRIPTION

[0018] As noted earlier, in applications that employ a fluid medium(rectangular) waveguide of very reduced height cross-section (i.e.,having a fixed width w and height h (where h<<w)), the substantialmismatch between the acoustic transmission properties of the acousticwave propagation medium (water) and those of the (piezoelectric)transducer typically creates a significant disparity between theacoustic and electrical impedance parameters of the piezo-electrictransducer and those of downstream and upstream components to which itis coupled.

[0019] This impedance mismatch problem is successfully obviated by meansof the acoustic impedance transformer structure diagrammaticallyillustrated in FIGS. 3 and 4, in which an acoustic impedance-matchingtransformer 40 is inserted between the relatively narrow acoustic wavecoupling aperture 34 of the waveguide channel 36 and a piezo-electrictransducer 50, to which a drive signal is applied from upstream RF drivesignal interface circuitry 60.

[0020] To compensate for this mismatch, the acoustic impedancetransformer 40 includes a first acoustic wave propagation element 41, inthe form of relatively dense, acoustic energy transmission material,such as metal (e.g., aluminum as a non-limiting example). The aluminumblock 41 has an acoustic impedance that differs from that of thepiezo-electric transducer 50 in a metric direction toward that of theacoustic propagation medium (water) of the waveguide. Namely, theacoustic impedance of the first transformer element is ‘stepped-down’from that of the piezo-electric transducer. Because of the difference,the relative spatial dimensions of the mating surfaces of the transducer50 and the aluminum block 41 are tailored by an amount that providesbroadband efficient coupling of acoustic energy (i.e., provide aneffectively impedance-matched acoustic interface) therebetween.

[0021] For the illustrated example, the ratio of the respective acousticimpedances of the piezo-electric transducer 50 and the aluminum block 40is 32.3/17.3; this means that to provide an efficient coupling ofacoustic energy between these two components, the surface area of theend face 42 of the aluminum block 41 engaging the acoustic energylaunching face 52 of transducer 50 should be a corresponding multiple of(32.2/17.3 times) the area of the acoustic energy-launching face 52 oftransducer 50. As a non-limiting example, therefore, assuming the samedimension (e.g., 5 mm) in the width direction of the various components,and with a piezo-electric transducer height on the order of 1.6 mm, theheight 43 of the end face 42 of the aluminum block 40 needs to be(32.3/17.3) times the height 53 of end face 52 of piezo-electrictransducer 50, or approximately 3 mm. For a 500 KHz system, the acousticwavelength in aluminum is 12 mm, so that the height of the aluminum isconsiderably less than the wavelength.

[0022] Although such dimensioning of the engaging Faces of thepiezo-electric transducer 50 and the aluminum block 41 serves to provideefficient coupling of acoustic energy from the transducer into thealuminum block, the cross-sectional area (3 mm×5 mm) of the aluminumblock at its acoustic energy receiving face 42 is considerably largerthan that (1 mm×5 mm) of the acoustic wave coupling aperture 34 of thewaveguide channel 36. In addition, event though the acoustic impedanceof the aluminum block 41 is less than the that of the piezo-electrictransducer, it is still considerably larger than that of the propagationmedium (e.g., water) of the waveguide channel 36.

[0023] To compensate for this front end mismatch, the aluminum block 41is physically tapering toward the acoustic input aperture 34 of thewaveguide channel. This has the effect of concentrating or focusing theacoustic energy traveling through the aluminum block, so as to achieve aprescribed acoustic energy power density at a reduced cross-section endface 44 of the aluminum block adjacent to the acoustic aperture of thewaveguide. The extent to which the acoustic energy is concentrated atthe end face 44 of the aluminum block 41 is determined in accordancewith difference between the acoustic impedance of the aluminum block anda further ‘stepped-down’ impedance of a second acoustic wave propagationelement 70, such as a section of plexiglass or the like, as anon-limiting example. This further impedance-transforming element isphysically sized to match the size of the acoustic input aperture 34 ofthe waveguide.

[0024] Plexiglas has an acoustic impedance (e.g., 3.16×10⁶ that is lessthan that of the tapered aluminum block, but greater than (approximatelytwice that of water (1.5×10⁶ the waveguide channel 36. Based upon theseparameters, the second (plexiglass) element 70 may be configured as aquarter-wave plate. Since the plexiglass element 70 has the samedimensions as the aperture 34 (i.e. a height of 1 mm), then to provide asubstantial impedance match between the aluminum block and theplexiglass section, the ratio of the area of the abutting end face 44 ofthe aluminum block 41 is set at a fraction of the area of the plexiglasselement 70. This fraction is based upon the ratios of the inherentacoustic impedances of the two elements and desired acoustic coupling ofthe increased power density provided at the end face 44 of the aluminumblock 71. In the present example, the height 45 of the second end face44 of the aluminum block 41 may have a height on the order of 0.38 mm.

[0025] Since the plexiglass element 70 has a cross-section correspondingto that of the acoustic input aperture 34 of the ATWL waveguide 30 andan acoustic impedance that is approximately twice that of the waveguidemedium (water), it is preferably configured as a quarter-wave platesection, so as to provide an acoustic impedance match between itsinterface with the reduced cross-section end of the tapered aluminumblock and the waveguide. The result of this cascaded series of acousticpropagation elements of successively decreasing or ‘stepped-down’acoustic impedance transform components—from the input face of taperedaluminum block to the exit face of the plexiglass quarter-wave plate—isto effectively match the acoustic impedance of the piezo-electrictransducer to that of the liquid acoustic traveling wave lens.

[0026] While we have shown and described an embodiment in accordancewith the present invention, it is to be understood that the same is notlimited thereto but is susceptible to numerous changes and modificationsas are known to a person skilled in the art, and we therefore do notwish to be limited to the details shown and described herein, but intendto cover all such changes and modifications as are obvious to one ofordinary skill in the art.

What is claimed:
 1. An arrangement for launching an acoustic wave into anarrow channel waveguide of an acousto-optic scanner, said narrowchannel acoustic waveguide containing an acoustic wave propagationmedium, upon which a scanned optical beam to be modulated by saidacoustic wave traveling through said narrow channel waveguide isincident, said narrow channel waveguide extending between a first endand a second end thereof, said arrangement comprising: an acoustictransducer having a first acoustic impedance and being operative togenerate an acoustic wave in response to the application of anelectrical drive signal thereto; and an acoustic impedance transformerhaving a second acoustic impedance different from said first acousticimpedance and different from a third acoustic impedance of said acousticwave propagation medium of said narrow channel waveguide, and having afirst surface to which said acoustic transducer is coupled, and a secondsurface coupled to said narrow channel waveguide, and wherein saidacoustic impedance transformer is physically configured to effectivelymatch said first acoustic impedance of said acoustic transducer withsaid third acoustic impedance of said acoustic wave propagation mediumof said narrow channel waveguide.
 2. An arrangement according to claim1, wherein said acoustic transducer has an electrical impedance matchedto that of drive circuitry supplying said electrical drive signalthereto.
 3. An arrangement according to claim 1, wherein said narrowchannel waveguide includes first and second generally parallel spacedapart walls through which said scanned optical beam passes, and thirdand fourth generally spaced apart walls, which intersect said first andsecond spaced apart walls and confine a liquid acoustic wave propagationmedium therebetween.
 4. An arrangement according to claim 3, whereinsaid separation between said third and fourth spaced-apart walls is lessthan one-half a wavelength, but greater than one-quarter wavelength ofsaid acoustic traveling wave.
 5. An arrangement according to claim 3,wherein said separation between said third and fourth spaced apart wallsdecreases from said first end of said narrow channel waveguide to saidsecond end of said narrow channel waveguide so as to maintain a constantacoustic power density of said acoustic wave traveling therealong.
 6. Anarrangement according to claim 1, wherein said acoustic impedancetransformer has a first end that is coupled to and is of a differentsize than a first surface of said transducer, and said acousticimpedance transformer has a second end that is coupled to an acousticinput aperture of said narrow channel waveguide.
 7. An arrangementaccording to claim 6, wherein said acoustic impedance transformercomprises a first acoustic wave propagation element having an acousticimpedance different from said second acoustic impedance, and a first endhaving a surface area different from that of the surface area of saidtransducer coupled thereto, and a second acoustic wave propagationelement having an acoustic impedance different from those of said firstacoustic wave propagation element, said acoustic transducer and saidnarrow channel waveguide, adjoining said first acoustic propagationelement and having a first end coupled to said narrow channel waveguide.8. An arrangement according to claim 7, wherein said first acoustic wavepropagation element has a second end of a surface area different thanand adjoining a second end of said second acoustic propagation element.9. An arrangement according to claim 8, wherein said first acousticpropagation element is tapered from said first end to said second endthereof.
 10. An arrangement according to claim 9, wherein said first endof said first acoustic propagation element has a surface area largerthan that of said acoustic transducer coupled thereto, and said secondend of said first acoustic propagation element has a surface area lessthan that of said second end of said second acoustic propagationelement.
 11. An arrangement according to claim 7, wherein said secondacoustic propagation element comprises an acoustic quarter-wave plate,said first end of which has a surface area proximate that of an endsurface of said narrow channel waveguide.
 12. For use with anacousto-optic scanner having a narrow channel acoustic waveguidecontaining a liquid acoustic wave propagation medium, upon which ascanned optical beam modulated by an acoustic traveling wave launchedthrough said narrow channel waveguide is incident, said narrow channelwaveguide extending from a first end to which an acoustic transducer iscoupled and a second end that terminates an acoustic traveling wavelaunched from said first end thereof, a method of coupling acousticenergy from said transducer into said first end of said narrow channelwaveguide comprising the steps of: (a) coupling said acoustic transducerto an acoustic impedance transformer having a first acoustic impedancedifferent from a second acoustic impedance of said transducer and athird acoustic impedance of said acoustic wave propagation medium ofsaid narrow channel waveguide; and (b) physically configuring saidacoustic impedance transformer to effectively match said second acousticimpedance of said transducer with said third acoustic impedance of saidacoustic wave propagation medium of said narrow channel waveguide.
 13. Amethod according to claim 12, wherein said acoustic transducer has anelectrical impedance matched to that of drive circuitry supplying anelectrical signal that stimulates said acoustic transducer to launch anacoustic wave into said acoustic impedance transformer.
 14. A methodaccording to claim 12, wherein said acoustic impedance transformer has afirst end that is coupled to and is of a different size than a firstsurface of said acoustic transducer, and a second end that is coupled toan acoustic input aperture of said narrow channel waveguide.
 15. Amethod according to claim 14, wherein said acoustic impedancetransformer comprises a first acoustic wave propagation element havingan acoustic impedance different from said second acoustic impedance ofsaid transducer and a first end, to which said transducer is coupled,said first end of said first acoustic wave propagation element having asurface area different from that of the surface of said transducercoupled thereto, and a second acoustic wave propagation element havingan acoustic impedance different from those of said first acoustic wavepropagation element, said transducer and said narrow channel waveguide,adjoining said first acoustic propagation element and having a first endcoupled to said narrow channel waveguide.
 16. A method according toclaim 15, wherein said first acoustic wave propagation element has asecond end of a surface area different than and adjoining a second endof said second acoustic propagation element.
 17. A method according toclaim 16, wherein said second acoustic propagation element comprises anacoustic quarter-wave plate having a surface area proximate that of saidacoustic aperture of said narrow channel waveguide.
 18. A methodaccording to claim 17, wherein narrow channel waveguide includes firstand second generally parallel spaced apart walls through which saidscanned optical beam passes, and third and fourth generally spaced apartwalls that intersect said first and second spaced apart walls, andconfining a liquid acoustic wave propagation medium therebetween.
 19. Amethod according to claim 18, wherein said separation between said thirdand fourth spaced-apart walls is less than one-half a wavelength, butgreater than one-quarter wavelength of said acoustic traveling wave. 20.A method according to claim 19, wherein said separation between saidthird and fourth spaced apart walls decreases from said first end ofsaid channel to said second end of said channel so as to maintain aconstant acoustic power density of said acoustic wave travelingtherealong.